1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus and, more particularly, to a photoelectric conversion apparatus having a switch array for converting a parallel input signal into a serial output signal.
2. Related Background Art
FIG. 1 is a circuit diagram schematically showing part of a conventional photoelectric conversion apparatus. Referring to FIG. 1 a photocurrent corresponding to an amount of incident line flows through photosensors Si (i=1, 2, . . . , m), and a charge is accumulated in a capacitor Ci. Subsequently, an H-level output is produced from a parallel output terminal Qi of a shift register 702, and sequentially turns on an analog switch SWti. When the analog switch SWti is turned on, the charge accumulated on the capacitor Ci is discharged through a common signal line 701 and the current is amplified by an amplifier 703. The amplified current is then output.
In FIG. 1, a switch means for resetting the capacitors Ci and the common signal line 701 is omitted. When the read access from the capacitor Ci is completed, the common signal line 701 is reset, and read access from the next capacitor Ci+1 starts.
However, the conventional photoelectric conversion apparatus poses the following problems. Since a noise component due to a gate-source capacitance of the analog switch SWti is output through the common signal line 701 and the amplifier 703, the S/N ratio of the serial output signal is impaired.
In such a conventional switch array apparatus, since the charge accumulated in the capacitor Ci is read out as a current, the S/N ratio of the output signal is impaired by variations in conductance gm of the analog switch SWti.
A noise component due to the gate-source capacitance of the analog switch SWti has a spike-like waveform because of current read access, and has a much higher frequency component than that of a readout signal. Therefore, the noise component cannot be eliminated even if a differential amplifier is used. Thus, a satisfactory S/N ratio cannot be obtained.
In the conventional switch array apparatus, when an output is read from the photosensors Si, a low signal current from each photosensor must be transmitted to the amplifier 703 through the common signal line 701. Therefore, the S/N ratio of the readout signal is decreased.